There are various naturally-occurring microbes which produce .beta.-1,3-glucan. For example, microbes such as those belonging to the genus Alcaligenes or Agrobacterium are known to extracellularly produce a .beta.-1,3glucan known as curdlan [New Food Industry, Vol. 20, p. 49 (1978), U.S. Pat. No. 3,754,925 etc.], and microbes such as those belonging to the genus Euglena are known to produce a .beta.-1,3-gl etc.).
Conventional production methods for .beta.-1,3-glucan based on cultivation of such microbes use as nitrogen sources various inorganic salts such as ammonium phosphate, ammonium sulfate and ammonium hydrochloride, urea, asparagine etc. [Agricultural Biological Chemistry, Vol. 30, pp. 764-769 (1966), U.S. Pat. No. 3,754,925, JP-A 60-58064 etc.]. When these inorganic salts are used, the pH of the medium is maintained at a prescribed level by means of pH regulation such as by addition of calcium carbonate to the medium or by addition of alkali ions such as those of caustic soda to the medium while monitoring the pH, since pH decreases during cultivation.
Conventional methods are known to pose problems such as the formation of sparingly soluble salts resulting from the reaction of the calcium carbonate added to the medium with an union such as the phosphate, sulfate or hydrochloride ion in a compound added as a nitrogen source. Another problem is the adverse effect of the alkali ion added for pH regulation on cultivation. In addition, if salts form in the culture broth, an additional operation or step is required to remove them in a purification process. All these phenomena are undesirable.